1. The Field of the Invention
The invention relates to a radiation-selective absorber coating comprising at least two barrier layers, an infrared reflective layer applied on the barrier layers, at least one absorption layer arranged above the infrared reflective layer and an antireflection layer arranged above the at least one absorption layer, wherein a first barrier layer consists of a thermally produced oxide. The invention also relates to an absorber tube having a radiation-selective coating of this type, and to a method of operating a parabolic trough collector containing absorber tubes of this type.
2. The Description of the Related Art
Customary absorber coatings consist of a layer which is reflective in the infrared spectral range and is applied on a substrate, in particular a metal tube, a cermet layer having a high absorptance in the range of the solar spectrum and a covering layer applied on the cermet layer, which is referred to as an antireflection layer and, because of the high refractive index of the cermet layer, is provided for reducing surface reflection from the cermet layer.
A fundamental aim of research efforts is to achieve an energy conversion yield that is as high as possible. The energy conversion yield is dependent, inter alia, on the coefficient of the absorptance a and the emissivity E. A high absorptance greater than 95% and a low emissivity (ε<10%) of the absorber coating is always the goal of research and development work.
Furthermore, the efficiency of the collector is determined by the temperature at which it is operated. From this standpoint, a temperature that is as high as possible is desired. However contrary to this the durability of the layer system of the absorber coating decreases with increasing operating temperature because of ageing and/or diffusion processes, as a result of which, for example, the absorptance of the cermet layer and the reflectivity of the reflective layer that reflects infrared radiation can significantly decline.
An absorber coating applied on a steel substrate comprises an antireflection layer composed of SiO2, a cermet layer, an infrared reflective layer of molybdenum and a diffusion barrier layer consisting of Al2O3 arranged between the infrared reflective layer and the substrate. This absorber coating is described in the article entitled “Solar selective absorber coating for high service temperatures, produced by plasma sputtering” by Michael Lanxner and Zvi Elgat, in SPIE, Vol. 1272, Optical Materials Technology for Energy Efficiency and Solar Energy Conversion IX (1990), pages 240 to 249. Stresses exist which have an adhesion-reducing effect or lead to destruction and lumping of the layer system within this sort of layer system.
DE 10 2004 010 689 B3 discloses an absorber with a radiation-selective absorber coating. This coating has a metal substrate, a diffusion barrier layer, a metallic reflection layer, a cermet layer and an antireflection layer. The diffusion barrier layer is an oxide layer consisting of oxidized components of the metal substrate.
Molybdenum is usually used for the reflective layers that are reflective in the infrared range. However, the reflection properties of a molybdenum layer are not optimal, and so it is desirable to use better reflective materials.
The operating temperature of known absorber tubes is 300° C. to 400° C. in a vacuum. For the reasons described above, a fundamental goal is to further increase the operating temperature, but without impairing, for example, the absorption properties of the cermet layer and the reflection properties of the infrared reflective layer.
Such endeavors are summarized in C. E. Kennedy, “Review of Mid- to High-Temperature Solar Selective Absorber Materials”, Technical Report of the National Renewable Energy Laboratory, July 2002 edition. This report discloses a layer structure composed of a ZrOxNy or ZrCxNy absorption layer and a layer of Ag or Al which is reflective in the IR range. This layer structure has improved thermal stability in air because of the introduction of an Al2O3 diffusion barrier layer. It was furthermore ascertained that the thermal stability of the infrared reflective layer under reduced pressure can be improved by introduction of a diffusion barrier layer below this layer. For this diffusion barrier layer, Cr2O3, Al2O3 or SiO2 are proposed as the layer material. The hope is to achieve stability of the silver reflective layer up to 500° C.
However, still more durable layers in conjunction with improved absorptance and emissivity are desirable.
Therefore, DE 10 2006 056 536 A1 describes a radiation-selective absorber coating comprising at least two barrier layers, an infrared reflective layer which is reflective in the IR range and is arranged on the barrier layers, an absorption layer arranged above the infrared reflective layer, and an antireflection layer arranged above the absorption layer, which has a high solar absorptance and a low thermal emissivity. Although the adhesion of this absorber coating is sufficient, it still needed further improvement.